UNIT NOT BLOWING WARM AIR-OR REACHING DESIRED TEMP

Question

My model number is not recognized but the correct one is WSKA-A024JK 000My problem is the same only about the cool air, if the temp outside gets warmer than about 60 degrees or so it seems the heat is working fine. If the temp outside is colder then set the unit on 70 it continuously runs. Set it at 69 and seems to shut off and on correctly. The air feels warm at first then blows cool. If the unit is set any higher it never reaches that temp and continuously runs. My electric bill went from around 135.00 to 456.00 from one month to the next. Breakers have been changed and the vents all seem to be connected and there are no holes. It is getting really annoying to hear this thing run constantly and not heat as other type units I have had. I do live in an older home with less insulation than a new home but we're working on that as well. Upstairs the heat that blows for that area is warm to hot and really works well. I'm disappointed and really need some advice.